Mobile telephones, for example, include a transceiver of which the receiver receives transmissions from a base station. The transmissions are usually in the form of frequency modulated signals utilising a high frequency carrier wave. Information to be transmitted may be voice or data information. In the former case, the audio signal is converted to digital form. Data information is normally in digital form. The digital information is chopped into packets of predetermined length and to each packet is added a header. The packets are usually transmitted in a time division multiplexed manner. Guassian Minimum Shift Keying (GMSK) is often used for transmissions of this kind. The bit stream of a packet, which may have been encoded in some manner, is passed through a Guassian filter. The effect of the filter is to modulate each information bit in dependence upon the preceding bits. The waveform so formed is used to modulate the high frequency carrier wave and is transmitted in its appropriate time slot.
In the receiver, the received r.f. signal is converted to baseband. If no interference is present, the baseband signal would only require decoding and demodulating to reconstitute the bitstream representing the transmitted information.
Particularly in built up areas, multipath reflections of the transmitted signal gives rise to multipath interference in which a last received echo may have been delayed by, for example, a time equal to four bit intervals. This multipath interference is known as "channel impulse response" and hence the received baseband signal is distorted by the channel impulse response.
It is known to include, in each transmitted packet header, a predetermined encoded and Guassian filtered bitstream. If the portion of each received distorted packet which includes this predetermined bitstream is compared in the receiver with an interference free representation of the same bitstream, the channel impulse response can be determined. The now-known channel impulse response enables the rest of the information in the packet to be interpreted on a probability basis. This may be effected using a Viterbi algorithm to compare the received distorted GMSK signal with all possible signals and to select, on the basis of probability, a "most likely" signal. Such selected most likely signal is then assumed to be the original undistorted signal.
Assuming only seven bit words in the transmitted data, an apparatus to effect a complete comparison, employing the Viterbi algorithm, would be complex, expensive, and of size and power requirements such that it would not constitute a commercial solution to the problem of multipath interference, particularly in receivers such as those used in mobile telephones.
It is an object of the present invention to provide an equaliser, for a radio receiver, wherein the aforesaid disadvantages are overcome.